OCEAN ELECTRONIC NAVIGATIONAL AIDS 33 



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Fig ORE 2-12. — Illustrating the directional characteristic of a coil antenna. 



available to the ship's navigator, consideration must be given to the location 

 of the direction finder loop antenna. This antenna should not be located 

 behind or near large metal objects such as vessel stacks, superstructure, 

 trunks, etc. In most vessels the choice of loop antenna location will be a 

 compromise due to necessity. The ideal location aboard any vessel would 

 be one where the loop antenna is well clear of all objects, permitting maximum 

 use and accuracy of the direction finder equipment. 



It is especially important that a radio direction finder have good selec- 

 tivity so as to be able to eliminate interference from other radio signals on 

 other frequencies when taking a bearing. For the usual needs of navigation 

 It is not essential that it be capable of taking radio bearings from great 

 distances. 



There are several other types of radio direction finders in other countries, 

 the most widely used of which is the fixed-loop (Bellini-Tosi) direction finder, 

 which employs a small rotatable search coil. All radio direction finders, 

 however, operate on the same basic principles. 



CONTINUOUS CARRIER MARINE RADIOBEACONS FOR AUTOMATIC 



RADIO DIRECTION FINDER USE 



In recent years automatic radio direction finders have made their appear- 

 ance, especially in aircraft. These devices, when manually tuned to the 

 desired station, will rapidly and automatically indicate the bearing of that 

 station. In order to provide the best possible navigational service, where re- 

 quired, the United States Coast Guard has modified 16 marine radiobeacon 

 stations for continuous-carrier, tone-keyed (1,020-cycle) operation for ADF 

 use. Two of the stations modified provide continuous service. These two 

 installations are outside the continental limits of the United States. The 

 remaining 14 stations provide service in the same manner as the conventional 

 marine radiobeacon, i. e., continuous carrier, in-sequence operation. This 

 type of service requires that the particular station share an assigned radio- 

 beacon frequency with two other stations. This is necessary in order to make 

 available to the user of the system a maximum number of radiobeacon sta- 

 tions operating in the limited radio-frequency spectrum available. The non- 

 automatic type of direction finder is not affected by the continuous-carrier, 

 tone-keyed type of transmission ; bearings may be obtained from such stations 

 without difficulty. 



OCEAN STATION VESSEL RADIOBEACON SERVICE 



The United States Coast Guard maintains ocean station vessels on a num- 

 ber of ocean stations in the Atlantic and Pacific for the primary purpose of 

 furnishing aeronautical weather information. One of the many auxiliary 

 duties of these vessels is to provide radiobeacon service to transoceanic air- 

 craft. The service provided is scheduled continuous-carrier, tone-keyed 

 signals, transmitted in the aeronautical radiobeacon band. Essentially the 

 equipment installed in the vessels is the same as that provided for coastal 

 marine radiobeacon stations. 



